Investigating the spatial accuracy of CBCT-guided cranial radiosurgery: A phantom end-to-end test study.

Purpose: To evaluate the spatial accuracy of a frameless cone-beam computed tomography (CBCT)-guided cranial radiosurgery (SRS) using an end-to-end (E2E) phantom test methodology.

Methods And Materials: Five clinical SRS plans were mapped to an acrylic phantom containing a radiochromic film. The resulting phantom-based plans (E2E plans) were delivered four times. The phantom was setup on the treatment table with intentional misalignments, and CBCT-imaging was used to align it prior to E2E plan delivery. Comparisons (global gamma analysis) of the planned and delivered dose to the film were performed using a commercial triple-channel film dosimetry software. The necessary distance-to-agreement to achieve a 95% (DTA95) gamma passing rate for a fixed 3% dose difference provided an estimate of the spatial accuracy of CBCT-guided SRS. Systematic (∑) and random (σ) error components, as well as 95% confidence levels were derived for the DTA95 metric.

Results: The overall systematic spatial accuracy averaged over all tests was 1.4mm (SD: 0.2mm), with a corresponding 95% confidence level of 1.8mm. The systematic (Σ) and random (σ) spatial components of the accuracy derived from the E2E tests were 0.2mm and 0.8mm, respectively.

Conclusions: The E2E methodology used in this study allowed an estimation of the spatial accuracy of our CBCT-guided SRS procedure. Subsequently, a PTV margin of 2.0mm is currently used in our department.